Method of geochemical prospecting



United States Patent METHOD OF GEOCHEMICAL PROSPECTING Norman D.Coggeshall, Verona, and William E. Hanson,

Pittsburgh, Pa., assignors to Gulf Research & Development Company,Pittsburgh, Pa., a corporation of Delaware No Drawing. ApplicationNovember 24, 1952, Serial No. 322,362

Claims. (Cl. 25.0-43.5)

This invention relates to a method of geochemical prospecting for hiddenhydrocarbon deposits by analyzing subsurface brines or other Waters forthe presence of dissolved hydrocarbon components, in particular aromatichydrocarbons and specifically benzene.

Heretofore, one method of geochemical prospecting has been conducted bymaking systematic collection of soil samples over an area to be mapped.The soil samples have been tested for a variety of components indicativeof the presence of hydrocarbon deposits. Methods of extracting thecomponents from the soil samples have included the use of organicsolvents or simply the application of heat to remove hydrocarbon gasestrapped in the soil samples. Among the components for which thesesamples have been tested are the lighter paraffinic hydrocarbons whichare presumed to have migrated from deep subsurface hydrocarbon deposits.One of the difliculties encountered with this method of prospecting isthat generally there is a wide Variation of the sorptive and retentivecapacities of the soil samples taken over a plurality of locations. Thisis due primarily to the differences in chemical and physical propertiesof the soil itself. A pattern of such concentration values may thus notgive a true indication of the proximity of hydrocarbon deposits. Anotherdifiiculty encountered is that some :of the lower molecular weightparaflinic hydrocarbons may arise from the recent decay of organic orvegetable matter instead of from petroleum deposits.

In other known methods of geochemical prospecting, techniques have beenused for detecting extremely small concentrations of crude hydrocarbonsdispersed in drilling fluids. These methods have involved subjecting thefluid or cuttings samples to ultraviolet light to produce fluorescenceby means of which it is possible to detect extremely smallconcentrations of crude oil which would ordinarily be invisible to theunaided eye. One of the problems which has been encountered in this typeof prospecting for crude oil deposits is that positive indications areobtained When the drilling fluid is contaminated with refined petroleumproducts, such as pipe-thread grease or lubricating oils.

The present invention is directed to overcome the problems encounteredin the type of geochemical prospecting which involves the sampling ofsoils or formation waters for the detection of parafi'inic hydrocarbonsas components indicative of the presence of hydrocarbon deposits and toovercome the difliculties involved in detecting hydrocarbon deposits byanalyzing formation Water samples obtained by drill-stem tests orbottom-hole samplers. This invention therefore has as an object thedetection of dissolved aromatic hydrocarbons in formation waters ascomponents related to the proximity of hydrocarbon deposits. Anotherobject of this invention is the detection of dissolved aromatichydrocarbons in formation waters by radiation from the electromagneticspectrum. A further object of this invention is the detection of benzenedissolved in formation brine waters. Further objects of this inventionwill become apparent from the subsequent description.

2,767,320 talented Oct. 16, 1956 "ice The objects of the presentinvention are achieved by a method of geochemical prospecting whereininformation concerning the proximity of hydrocarbon deposits or sourcebeds is derived by collecting aqueous subterranean samples from prospectareas and wildcat wells and extracting the hydrocarbon componentsdissolved in these samples with a suitable extracting agent anddetermining the concentration of these hydrocarbons by the use ofelectromagnetic radiation. The presence of these dissolved hydrocarbonsis an indication that these subterranean waters have been in contactwith or are near a petroliferous deposit.

Aromatic hydrocarbons are universal components of crude petroleum andfurthermore this class of hydrocarbons is more soluble in water or brinethan any other type of hydrocarbon which occurs in crude petroleum.Benzene and a considerable number of benzene homologues have beenrecognized in crude petroleum. The best established are benzene,toluene, the three xylenes, ethylbenzene, napthalene and the two methylnaphthalenes. The actual percentages of these components depend on thegeographical area where the petroleum is found, but they are alwayspresent. The aqueous solubility of these aromatic hydrocarbons found inpetroleum decreases rapidly with an increase in molecular weight. Forthis reason, the lower molecular weight members of this series, such asbenzene, are favored as criteria for the proximity of petroleumdeposits. This is based on the observation that brines or other waterswhen brought into contact with petroleum will preferentially dissolveout a portion of the aromatics and particularly benzene and retain themin solution, although the waters may have been later physicallyseparated from the crude petroleum.

A number of investigations have been carried out in an attempt todevelop a simple and positive method of distinguishing .between thehydrocarbons from pipe-thread grease or other lubricantsthat may havecontaminated sample waters from drill-stem tests or bottom-hole samplersand hydrocarbon from crude oil as they occur in the brines. The basis ofthis distinction lies in the fact that certain hydrocarbons such asbenzene are more soluble in brine than any of the other constituents ofcrude oil. Furthermore, since benzene has a low boiling point it isvirtually absent from most refined products used around a drilling rigwith the exception of light fuel oils and gasoline. It is thereforepossible that these light fuel oils, gasolines or even natural gas whichare commonly used as fuels for powering drilling equipment, may containlight aromatics such as benzene. Precautions should therefore be takenthat contaminants from these sources are not present or if they arepresent they must be capable of removal. If a well or drill hole iscontaminated with benzene from a source other than sub-surface waters,the well can be flowed or pumped for a time suflicient to remove allcontaminated brine, so that all the benzene from the fresh brine wouldarise from the contact of the brine with underground crude hydrocarbondeposit. However, the sources of contamination mentioned above are notlikely to occur unless careless procedures are used. In actual practiceon a series of tests on several wells we have found that the wells gaveno evidence of contamination from such sources and we believe that suchspecial precautions or preliminary swabbing prior to obtaining testsamples are unnecessary.

By measuring the concentration of dissolved aromatic hydrocarbons,specifically that of dissolved benzene, in fluid samples obtained forinstance from wildcat wells, even though considered to be dry holes, itis possible to determine whether a particular formation is completelybarren or whether nearby portions may be petroliferous. Such informationobtained by detecting aromatic hydrocarbons in the subsurface waters orbrines from wildcat 3 wells may guide further drilling activity in thearea. Benzene in varying concentrations has been found in tests on dryholes and increasing benzene concentration is interpreted as indicatingthe direction toward petroleum deposits.

In practicing our invention any suitable method of extracting aromatichydrocarbons and particularly benzene as the component for analysis maybe employed. Solvent extraction of the brine or other. waters withsubsequent distillation to further concentrate and remove interferingmaterialfrom the aromatic hydrocarbons, followed by infrared orultraviolet spectroscopic examination of the extract, permits thedetection of aromatic components in the brine in concentrations of lessthan one part per million.

A method based on our procedure to be discussed below and using a strongultraviolet absorption band of benzene has given positive results andthere have been no indications of benzene whatever in synthetic brinesgiven prolonged exposure to pipe-thread grease and lubricating oil.

The samples for analysis may comprise formation waters obtained byswabbing, drill-stem test fluids obtained from a drill-stem test,or'samples or underground waters obtained by an convenient means. Thepreferred method of analyzing the water samples for benzene content willnow be described.

If the sample is a swab water, it may be extracted directly afterfiltering. If it is a mixture of water and mud such as from a drill-stemtest, the water for testing may be the supernatant liquid aftersettling, or satisfactory water may be obtained from the mixture bycentrifuging, filtration, or a combination of the two.

A preferred procedure for this invention is to extract a definite volumeof the sample water with aromatic-free iso-octane(2,2,4-trimethylpentane). In the extraction a two-liter sample isconveniently used. The water is placed in a glass container with 25 ml.of iso-octane (2,2,4-trimethylpentane) of spectroscopic grade. This isthen vigorously agitated in a mechanical shaker for 30 minutes. Afterthe two phases separate,'the iso-octane may in some cases be recovereddirectly by means of a pipette. In oher cases emulsions of varyingstrength will be obtained. These may be broken chemically or by means ofa centrifuge. The iso-octane will contain a major portion of the totalamount of benzene originally present in the sample water.

In the preferred form of our invention in which benzene is to bedetected, we prefer to concentrate the hem zene from the extract bydistillation. Subsequently a selected fraction of the distillation isquantitatively analyzed for benzene by ultraviolet-absorptionspectroscopy. Chromatographic fractionation, however, may also be employed for concentration of the benzene from the extract, and should asuitable colourimetric method for determination of benzene becomeavailable such method may be employed.

The iso-octane recovered from the extraction is measured and charged toa still with a still pot volume of approximately fifty ml. and with anefiiciency equivalent to about five theoretical plates. The charge isheated slowly until condensation is observed at the take-off point. The

. still is then allowed to operateat total reflux for one-half hour. Atthis point the sample is continuously removed until 3 mLof distillatehas accrued. This 3 ml. will contain most of the benzene in the originalcharge.

The distillate of the above step is then examined in an 7 calculatedfrom the observed benzene content in the disinterfere with itsspectroscopic examination.

ferred procedure referred to above the refluxing time is tillate and theratio of distillate volume to still-charge volume. The amount of benzenein the original water sample is then calculated from the benzeneconcentration in the charge to the still, the ratio of the volume of isooctane used for extraction to the volume of the original water sample,and a correction factor discussed below. If less than 25 ml. ofiso-octane are recovered'frcm the extraction, it is assumed that thebenzene is uniformly distributed between the recovered and non-recoveredoctane. As the extraction and distillation steps do not yield completerecovery it is necessary to evaluate a correction factor. This is doneby introducing known concentrations of benzene into water. These samplesare then extracted and processed as above. The results may thus be usedto provide the necessary correction factor.

The distillation step in our procedure is importantin effecting theremoval of material which would interfere with the spectroscopicexamination of the benzene extract. In the extraction step describedabove the benzene is transferred from the water phase to the hydrocarbonphase and as a consequence is concentrated in a smaller volume ofliquid. The distillation step serves a twofold purpose and for thisreason it is believed important. first purpose is to remove any materialthat may interfere with the spectroscopic examination. The secondpurpose of the distillation step is to further concentrate the benzenein the extract to a smaller volume. It is essential, therefore, that thecharge be heated slowly and that it should be refluxed until it iscertain that most of the henzene can be removed free from the materialsthat would In our precedure in view of the fact that other methods maybe practiced. Chromatographic fractionation for example affords thebasis for a very elfe'ctive method of concen- V trating and separatingmixtures, and this method may be used in the detection and determinationof benzene and other aromatic hydrocarbons occurring in subsurfacewaters which have been in contact With buried hydrocarbon deposits.

The numerical values of volumes of water and solvent, of extractiontime, of still volume, distillation time, theorectical plates, andvolume of distillate used are not critical and may be varied. The onlyrequirement is that once a set of conditions are established that it beadhered to forall calibration and sample runs.

One of the advantages of our invention in using formation waters as thesampling media is that they are ultraviolet spectrophotometer withradiation whose wave The remore easily handled and sampled than soilsamples or soil gases. Another advantage is that there is no evidence inthe literature that benzene is a product of recent vegetable or animaldecomposition and further that benzene is virtually absent fromcontaminants arising from lubrieating oils and greases employed indrilling machinery.

Having thus described our invention in terms of certain specificembodiments, we intend to cover all changes and modifications of theexample of the invention herein chosen for purposes of the disclosure,which do not constitute departure from the spirit and scope of theinvention.

What we claim as our invention is:

1. In the art of geochemical prospecting for buried hydrocarbon depositswhich comprises collecting aqueous subterranean samples in a prospectarea and analyzing these for hydrocarbon components indicativev of thepresence of hydrocarbon deposits, the irnprovement'which The Y .5comprises extracting the aromatic components dissolved in said aqueoussamples, and testing the extract for the presence 'of aromatichydrocarbons as a measure of the relative amounts of said aromatichydrocarbons originally present in said aqueous samples, ,andcorrelating these results as an indication of the proximity of buriedhydrocarbon deposits.

2. In the art of geochemical prospecting for petroliferous depositswhich comprises collecting samples of subsurface formation waters in aprospect area and analyzing these for hydrocarbon components indicativeof the presence of petroliferous deposits, the improvement whichcomprises extracting the dissolved hydrocarbon components in saidsamples and passing electromagnetic radiation through the extractcontaining the dissolved hydrocarbons for the purpose of detecting thepresence of aromatic hydrocarbons identified by their characteristicpattern of absorption bands, and determining their concentration in theoriginal water sample as an indication of the nearness of petroliferousdeposits.

3. The method of exploration for petroleum deposits which comprises asystematic collection of brine water samples from a prospect area,extracting the dissolved hydrocarbon content and passing ultravioletradiation through the extract containing the dissolved hydrocarbons forthe purpose of detecting the presence of aromatic hydrocarbonsidentified by their characteristic pattern of absorption bands, anddetermining their concentration in the original brine samples as anindication of the proximity of petroleum deposits.

4. In the art of geochemical prospecting for buried hydrocarbon depositswhich comprises collecting aqueous subterranean samples in a prospectarea and analyzing these for hydrocarbon components indicative of thepresence of hydrocarbon deposits, the improvement which comprisesextracting the dissolved aromatic components in said samples, anddetermining the concentration of benzene in the original samples as acomponent indicative of the presence of hydrocarbon deposits.

5. In the art of geochemical prospecting for petroliferous depositswhich comprises a systematic collection of samples of subsurfaceformation waters in a prospect area and analyzing these for hydrocarboncomponents indicative of the presence of petroliferous deposits, theimprovement which comprises extracting the dissolved hydrocarboncomponents in said samples, pass ing electromagnetic radiation throughthe extract containing the dissolved hydrocarbons for the purpose ofdetecting the presence of benzene as identified by its characteristicpattern of absorption bands as a component indicative of the proximityof petroliferous deposits.

6. The method of exploration for petroleum deposits which comprises asystematic collection of brine water samples over a prospect area,extracting the dissolved aromatic content therein and passingultraviolet radiation through the extract containing the dissolvedaromatic hydrocarbons in solution for the purpose of detecting thepresence of benzene identified by its characteristic absorption bands,and determining the concentration of benzene in the original brinesample as an indication of the proximity of petroleum deposits.

7. In the art of geochemical prospecting for buried hydrocarbon depositswhich comprises collecting aqueous subterranean samples in a prospectarea and analyzing these for hydrocarbon components indicative of thepresence of hydrocarbon deposits, the improvement which comprisesextracting the dissolved aromatic components from said aqueous samples,passing ultraviolet radiation through the extract containing thearomatic hydrocarbons in solution for the purpose of detecting thepresence of benzene identified by its characteristic absorption bandsand for determining its concentration in the original samples, andcorrelating these results as an indication of the proximity of buriedhydrocarbon deposits.

8. In the art ofgeochemieal prospecting for petroliferous deposits whichcomprises collecting samples of formation waters in a prospect area andanalyzin these for hydrocarbon components indicative of the presence ofpetroliferous deposits, the improvement which comprises extracting thedissolved benzene 'in said samples with a suitable non-aromatic solventof spectroscopic grade, concentrating the benzene contained in said,solvclltla ld passing ultraviolet radiation through the solventcontaining the benzene to determine the concentration of benzene in theoriginal samples as an indication of the nearness of petroliferousdeposits.

9. In the art of geochemical prospecting for petroliferous depositswhich comprises collecting samples of formation water in a prospect areaand analyzing these for hydrocarbon components indicative of thepresence of petroliferous deposits, the improvement which comprisesextracting the dissolved benzene in said samples with2,2,4-trimethylpentane, concentrating the benzene contained in saidsolvent and passing ultraviolet radiation of wave length in the region220 to 280 millimicrons through the solvent containing the benzene todetermine the concentration of benzene in the original samples as anindication of the nearness of petroliferous deposits.

10. The method of exploration for petroleum deposits which comprisescollecting samples of brine waters from wells, extracting the benzenecontent dissolved therein, passing ultraviolet radiation of wave lengthin the region 220 to 280 millimicrons through the extract containing thebenzene and determining the resulting absorption spectrum, theconcentration of benzene as a component indicative of the presence ofpetroleum deposits.

11. The method of detecting subsurface petroleum deposits by analyzingsubsurface brines which comprises extracting the aromatic componentsdissolved in the brine samples, passing ultraviolet radiation throughthe extract containing the aromatics, and determining the concentrationof the aromatic hydrocarbon components in the samples as in indicationof the proximity of petroleum deposits.

12. The method of detecting subsurface petroleum deposits by analyzingsubsurface brines which comprises extracting the benzene dissolved inthe brine samples, passing ultraviolet radiation through the extractcontaining the benzene, determining from the resulting absorptionspectrum the concentration of benzene as an indication of the proximityof petroleum deposits.

13. The method of detecting subsurface petroleum deposits by analyzingsubsurface brines which comprises extracting the benzene dissolved inthe brine samples with 2,2,4-trimethylpentane, passing ultravioletradiation of wave length in the region 220 to 280 millimicrons throughthe extract containing the benzene and determining through itsabsorption spectrum the concentration of benzene as an indication of theproximity of petroleum deposits.

14. The method of detecting subsurface petroleum deposits by analyzingsubsurface brines which comprises extracting the benzene dissolved inthe brine samples with 2,2,4-trimethylpentane, passing ultravioletradiation of wave length in the region 220 to 280 millimicrons throughthe extract containing the benzene, measuring the absorption spectrum ofthe extract for said radiation whereby the concentration of benzene inthe brine sample may be determined, and correlating the benzeneconcentration with the source location of the brine sample.

15. The method of exploration for petroleum deposits which comprises asystematic collection of subsurface water samples over a prospect area,extracting the benzene dissolved in the water sample with2,2,4-trirnethylpentane, distilling the hydrocarbon extract to furtherconcentrate benzene and to remove spectrally-interfering materials, andpassing ultraviolet radiation of wave length in the region of 220 to 280millimicrons through the extract containing the benzene, and determiningthrough its absorption spectrum the concentration of benzene as an2,500,213 indication of the proximity of petroleum deposits.

References Cited in the file of this patent "5 114,477

UNITED STATES PATENTS 2,403,631 Brown July 9, 1946 FOREIGN PATENTS V 7Australia Jan. 15, 1942

3. THE METHOD OF EXPLORATION FOR PETROLEUM DEPOSITS WHICH COMPRISES ASYSTEMATIC COLLECTION OF BRINE WATER SAMPLES FROM A PROSPECT AREA,EXTRACTING THE DISSOLVED HYDROCARBON CONTENT AND PASSING ULTRAVIOLENTRADIATION THROUGH THE EXTRACT CONTAINING THE DISSOLVED HYDROCARBONS FORTHE PURPOSE OF DETECTING THE PRESENCE OF AROMATIC HYDROCARBONSIDENTIFIED BY THEIR CHARACTERISTIC PATTERN OF ABSORPTION BANDS, ANDDETERMINING THEIR CONCENTRATION IN THE ORIGINAL BRINE SAMPLES AS ANINDICATION OF THE PROXIMITY OF PETROLEUM DEPOSITS.